Electrochromism: a useful probe to study algal photosynthesis

In photosynthesis, electron transfer along the photosynthetic chain results in a vectorial transfer of protons from the stroma to the lumenal space of the thylakoids. This promotes the generation of an electrochemical proton gradient (Δμ _H ⁺ ), which comprises a gradient of electric potential (ΔΨ) and of proton concentration (ΔpH). The Δμ _H ⁺ has a central role in the photosynthetic process, providing the energy source for ATP synthesis. It is also involved in many regulatory mechanisms. The ΔpH modulates the rate of electron transfer and triggers deexcitation of excess energy within the light harvesting complexes. The ΔΨ is required for metabolite and protein transport across the membranes. Its presence also induces a shift in the absorption spectra of some photosynthetic pigments, resulting in the so-called ElectroChromic Shift (ECS). In this review, we discuss the characteristic features of the ECS, and illustrate possible applications for the study of photosynthetic processes in vivo.     

Membrane electricity as a convertible energy currency for the cell

The role of transmembrane electric potential difference (delta psi) in mitochondria, chloroplasts, and bacteria has been considered. Since the electric capacitance of membranes is much lower than the pH buffer capacitance of water phases, delta psi proves to be the primary form of energy produced by generators of electrochemical H+ potential difference (delta mu-H). There are 11 distinct types of delta mu-H-generating systems in coupling membranes, involved in respiratory and light-dependent electron and proton transfer, as well as in ATP and PP1 hydrolysis and synthesis. Bacteriorhodopsin is the simplest delta mu-H generator. However, even in this case, the molecular mechanism of delta psi production remains obscure. Many types of work can be supported by delta mu-H with no ATP involved so that delta mu-H proves to be not only a transient intermediate of oxidative and photosynthetic phosphorylation but also a convertible energy currency for the cell. Among the delta mu-H-supported activities, mechanical work was recently demonstrated. It can be exemplified by the motility systems of (i) flagellar bacteria and (ii) blud--green algae. As was found in multicellular cyanobacteria, delta mu-H can be used for a power transmission over distances as long as 1 mm. It seems to be probable that in large cells of eukaryotes (e.g., in muscle fibers) giant mitochondria may serve as power-transmitting structures. Na+--K+ gradients can be used to stabilize delta mu-H in bacteria. It is suggested that the primary function of unequal distribution of these cations between the microbial cell and the medium is delta mu-H buffering.     

光合作用对光和CO2响应模型的研究进展

光合作用对光和CO₂响应模型是研究植物生理和植物生态学的重要工具, 可为植物光合特性对主要环境因子的响应提供科学依据。该文综述了当前光合作用对光和CO₂响应模型的研究进展和存在的问题, 并在此基础上探讨了这些模型的可能发展趋势。光合作用涉及光能的吸收、能量转换、电子传递、ATP合成、CO₂固定等一系列复杂的物理和化学反应过程。光合作用由原初反应、同化力形成和碳同化3个基本过程构成, 任一个过程均可对光合作用速率产生直接的影响。光合作用对光响应模型只涉及光能的转换, 而光合作用的生化模型包含了同化力形成和碳同化这两个基本过程。把光合作用的原初反应, 即把参与光能吸收、传递和转换的捕光色素分子的物理参数(如捕光色素分子数、捕光色素分子光能吸收截面、捕光色素分子处于激发态的平均寿命等)结合到生化模型中, 可能是今后光合作用对光响应机理模型的发展方向。     

Flip-flop of phospholipids in proteoliposomes reconstituted from detergent extract of chloroplast membranes: kinetics and phospholipid specificity

Eukaryotic cells are compartmentalized into distinct sub-cellular organelles by lipid bilayers, which are known to be involved in numerous cellular processes. The wide repertoire of lipids, synthesized in the biogenic membranes like the endoplasmic reticulum and bacterial cytoplasmic membranes are initially localized in the cytosolic leaflet and some of these lipids have to be translocated to the exoplasmic leaflet for membrane biogenesis and uniform growth. It is known that phospholipid (PL) translocation in biogenic membranes is mediated by specific membrane proteins which occur in a rapid, bi-directional fashion without metabolic energy requirement and with no specificity to PL head group. A recent study reported the existence of biogenic membrane flippases in plants and that the mechanism of plant membrane biogenesis was similar to that found in animals. In this study, we demonstrate for the first time ATP independent and ATP dependent flippase activity in chloroplast membranes of plants. For this, we generated proteoliposomes from Triton X-100 extract of intact chloroplast, envelope membrane and thylakoid isolated from spinach leaves and assayed for flippase activity using fluorescent labeled phospholipids. Half-life time of flipping was found to be 6 ± 1 min. We also show that: (a) intact chloroplast and envelope membrane reconstituted proteoliposomes can flip fluorescent labeled analogs of phosphatidylcholine in ATP independent manner, (b) envelope membrane and thylakoid reconstituted proteoliposomes can flip phosphatidylglycerol in ATP dependent manner, (c) Biogenic membrane ATP independent PC flipping activity is protein mediated and (d) the kinetics of PC translocation gets affected differently upon treatment with protease and protein modifying reagents.     

Nucleotide transport in Rhodobacter capsulatus.

Cytoplasmic membrane vesicles isolated from the gram-negative photosynthetic bacterium Rhodobacter capsulatus catalyzed the transport of nucleotides. No transport occurred in the intact bacteria unless they were pretreated with EDTA. The transport rate was measured by incorporation of radioactive phosphate into externally added ADP or by incorporation of nonradioactive phosphate into added labeled ADP. The catalytic activities which utilized the added ADP were photosynthetic ATP synthesis, Pi-ADP exchange, and adenylate kinase. These activities were shown to occur on the cytoplasmic side of the internal membrane. The products were found in the outer medium. The rate of nucleotide transport across the membranes was comparable to the rate of photophosphorylation. These results indicated that nucleotides can be transported across the cytoplasmic membrane but not across the outer membrane of the native R. capsulatus cell. Therefore, by analogy to the mitochondrial ATP-ADP translocator, the exchange might function as an energy transfer system to the periplasm of these bacteria.     

Photoelectric currents across planar bilayer membranes containing bacterial reaction centers. Response under conditions of single electron turnover.

Light-induced electric current and potential responses have been measured across planar phospholipid membranes containing reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides. Under conditions in which the reaction centers are restricted to a single electron turnover, the responses can be correlated with the light-induced electron transfer reactions associated with the reaction center. The results indicate that electron transfer from the bacteriochlorophyll dimer to the primary ubiquinone molecule, and from ferrocytochrome c to the oxidized dimer occur in series across the planar membrane. Electron transfer from the primary to secondary ubiquinone molecule is not electrogenic.     

Electrical-to-mechanical coupling in purple membranes: membrane as electrostrictive medium

In this paper, we present acousto-electrical measurements performed on dry films of purple membranes (PM) of Halobacterium salinarium. The purpose of these measurements is to determine the relation between mechanical and electrical phenomena in bacteriorhodopsin and to define the role of the protein in the proton transfer process. Electrical-to-mechanical coupling in PMs manifests itself as direct and inverse piezoelectric effects. Measurements performed on the samples with different degrees of PM orientation and at various values of the externally applied cross-membrane electric field indicate that piezoelectric phenomena in PMs arise from the electric asymmetry of the membranes, i.e., they originate from electrostriction. Experiments with samples made of oriented PMs allow estimation of the value of the intrinsic cross-membrane electric field, which is approximately 10(8) V/m. A hypothetical model of PM is presented where the electrical-to-mechanical coupling is suggested to be the main driving force for the proton translocation against the Coulomb forces acting in the membrane.     

光系统Ⅱ的结构与功能以及光合膜对环境因素的响应机制

光合膜是地球上捕获、转换和利用太阳能的关键场所,光合膜的活动所提供的能源、粮食及氧气,是人类世界赖以生存的基础。经过35亿年的进化,光合膜已经进化成了一个高度精密的结构,色素分子高密度结合并合理排列,具有高精度的能级耦联网络和高效率的能量传递系统,这使得光合膜成为自然界中能够最高效地吸收和传递太阳能、并能在常温常压下高效地将太阳能转换成化学能和还原势的色素蛋白复合体体系。由于这一特性,光合膜被认为是最有潜力的固定太阳能的新材料,并为研究新型光电转换器件提供了新思路和新理论。因此,长期以来,光合膜的结构-功能关系研究及其功能模拟,特别是执行固定和转化太阳能第一步的光系统Ⅱ,在新能源的利用中吸引了大量的研究力量,取得了突飞猛进的进展。本文总结了近年来关于光系统Ⅱ的结构与功能,以及光合膜对环境的感应和功能调节机制等方面的研究进展。     

Evaluation of the Specific Roles of Anions in Electron Transport and Energy Transfer Reactions in Photosynthesis

Ionic environment is important in regulating photosynthetic reactions. The roles of cations, Mn²⁺, Mg²⁺, Ca²⁺, Na⁺, and K⁺ as cofactors in electron transport, energy transfer, phosphorylation, and carbon assimilation are better known than the roles of anions, except for chloride and bicarbonate. Only a limited information exists on the roles and effects of nitri formate, sulphate, and phosphate. In this review, we evaluate and highlight the roles of some specific anions on electron transport as well as on excitation energy transfer processes in photosynthesis. Anions exert significant effects on thyla membrane conformation and membrane fluidity, possibly by redistributing the thylakoid membrane surface charges. The anion/cation induced phase transitions in the hydrophilic domains of the thylakoid membranes are probably responsible for the various structural and co-related functional changes under stress. Anions are also important in regulation of energy distribution between the two photosystems. Anions do not only divert more energy from photosystem (PS) 2 to PS1, but can also reverse the effect of cations on energy distribution in a valence-dependent manner. Anions affect also the structure of the photosynthetic apparatus and excitation energy distribution between the two photosystems.     

Energy transduction in photosynthetic bacteria. VIII. Activation of the energy-transducing ATPase by inorganic phosphate.

ATPase activity and ATP-induced energization of photosynthetic membranes from Rhodopseudomonas capsulata are stimulated by phosphate; the maximum stimulatory effect occurs at a concentration between 1 and 2 mM. The sensitivity of the ATPase to oligomycin increases in the presence of phosphate since all the Pi-stimulated activity is inhibited by this antibiotic. Aurovertin, which has no effect on ATPase in the absence of phosphate, inhibits completely the activity elicited by this anion. The addition of Pi induces a substantial increase in the V of ATPase activity without changing the affinity of the enzyme for ATP or ADP. Arsenate, at the same concentrations, produces effects very similar to those of phosphate. The stimulation by arsenate of the transfer of energy from ATP to the membrane suggests a non-hydrolytic role of this anion as a modifier of the ATPase activity.     

Phycobilisomes from the mutant cyanobacterium Synechocystis sp. PCC 6803 missing chromophore domain of ApcE

Phycobilisome (PBS) is a giant photosynthetic antenna associated with the thylakoid membranes of cyanobacteria and red algae. PBS consists of two domains: central core and peripheral rods assembled of disc-shaped phycobiliprotein aggregates and linker polypeptides. The study of the PBS architecture is hindered due to the lack of the data on the structure of the large ApcE-linker also called L_CM. ApcE participates in the PBS core stabilization, PBS anchoring to the photosynthetic membrane, transfer of the light energy to chlorophyll, and, very probably, the interaction with the orange carotenoid protein (OCP) during the non-photochemical PBS quenching. We have constructed the cyanobacterium Synechocystis sp. PCC 6803 mutant lacking 235 N-terminal amino acids of the chromophorylated PBL_CM domain of ApcE. The altered fluorescence characteristics of the mutant PBSs indicate that the energy transfer to the terminal emitters within the mutant PBS is largely disturbed. The PBSs of the mutant become unable to attach to the thylakoid membrane, which correlates with the identified absence of the energy transfer from the PBSs to the photosystem II. At the same time, the energy transfer from the PBS to the photosystem I was registered in the mutant cells and seems to occur due to the small cylindrical CpcG2-PBSs formation in addition to the conventional PBSs. In contrast to the wild type Synechocystis, the OCP-mediated non-photochemical PBS quenching was not registered in the mutant cells. Thus, the PBL_CM domain takes part in formation of the OCP binding site in the PBS.     

The Rhodopseudomonas viridis photosynthetic membrane: arrangement in situ

The organization of photosynthetic membranes in the cytoplasm of the photosynthetic bacterium Rh. viridis has been examined by several techniques for electron microscopy. Thin sections of membrane stacks show that the regular lattice of membrane subunits reported in other studies can be observed in thin section. Tilting of sections in the electron microscope shows that the regular lattices of several membranes overlap in a way that suggests they are in register with each other. This observation can be confirmed by freeze-fracture images in which a regular arrangement of membrane lattices can be observed, each perfectly aligned.Analysis of the spacings of membrane pairs shows that the photosynthetic membranes of Rh. viridis are very closely apposed. The mean diameter of two membranes is 160A, and the average space between two such membranes is only 42A. When a recently developed atomic level model of Rh. viridis reaction center is superimposed against these spacings, each reaction center extends from the surface of its respective membrane far enough to make contact with an apposing membrane. The limited free space between membranes and regular alignment of lattices has a number of implications for how this membrane is organized to carry out the process of energy transfer.     

Photosynthetic Properties of ac-31, a Mutant Strain of Chlamydomonas reinhardi Devoid of Chloroplast Membrane Stacking

A pale-green mutant strain of Chlamydomonas reinhardi, ac-31, is characterized by the absence of any stacking of its chloroplast membranes. The capacity for photosynthetic electron transport, phosphorylation, and CO₂ fixation in ac-31 is substantial, and it is concluded that these photosynthetic activities occur within the single membrane. The photosynthetic capacities of wild type and ac-31 as a function of increasing light intensity are compared. Saturation is attained at higher light intensities in ac-31, and the kinetics of the 2 sets of curves are distinctly different. The possibility that energy transfer is enhanced by membrane stacking is suggested by these results. The repeatedly-observed correlation between reduced stacking and disfunctional Photosystem II activities is discussed in view of the observation that ac-31 has no stacking but retains a functional Photosystem II.     

Finite element analysis of bone remodelling with piezoelectric effects using an open-source framework

Bone tissue exhibits piezoelectric properties and thus is capable of transforming mechanical stress into electrical potential. Piezoelectricity has been shown to play a vital role in bone adaptation and remodelling processes. Therefore, to better understand the interplay between mechanical and electrical stimulation during these processes, strain-adaptive bone remodelling models without and with considering the piezoelectric effect were simulated using the Python-based open-source software framework. To discretise numerical attributes, the finite element method (FEM) was used for the spatial variables and an explicit Euler scheme for the temporal derivatives. The predicted bone apparent density distributions were qualitatively and quantitatively evaluated against the radiographic scan of a human proximal femur and the bone apparent density calculated using a bone mineral density (BMD) calibration phantom, respectively. Additionally, the effect of the initial bone density on the resulting predicted density distribution was investigated globally and locally. The simulation results showed that the electrically stimulated bone surface enhanced bone deposition and these are in good agreement with previous findings from the literature. Moreover, mechanical stimuli due to daily physical activities could be supported by therapeutic electrical stimulation to reduce bone loss in case of physical impairment or osteoporosis. The bone remodelling algorithm implemented using an open-source software framework facilitates easy accessibility and reproducibility of finite element analysis made.

     

Energy transduction in photosynthetic bacteria. VIII. Activation of the energy-transducing ATPase by inorganic phosphate

ATPase activity and ATP-induced energization of photosynthetic membranes from Rhodopseudomonas capsulata are stimulated by phosphate; the maximum stimulatory effect occurs at a concentration between 1 and 2 mM.The sensitivity of the ATPase to oligomycin increases in the presence of phosphate since all the P_i-stimulated activity is inhibited by this antibiotic. Aurovertin, which has no effect on ATPase in the absence of phosphate, inhibits completely the activity elicited by this anion.The addition of P_i induces a substantial increase in the V of ATPase activity without changing the affinity of the enzyme for ATP or ADP.Arsenate, at the same concentrations, produces effects very similar to those of phosphate. The stimulation by arsenate of the transfer of energy from ATP to the membrane suggests a non-hydrolytic role of this anion as a modifier of the ATPase activity.     

Mechanical and functional aspects of membrane skeletons

Membrane skeletons can be characterized as cytoskeletal structures lying parallel to the bilayer part of cellular and organelle membranes. Typical examples are spectrin network and actin-myosin cortex. We approach the problem of elucidating the function of membrane skeletons by theoretically analyzing mechanical models of the cellular behavior. Membranes of different physical and chemical properties are considered. In erythrocytes and some organelles membrane bilayers are smooth and simply underlaid or overlaid by membrane skeletons. It is argued that there the role of a membrane skeleton is, either, to keep the membrane composition laterally homogeneous as it is in the case of the erythrocyte, or, that it is involved in the processes of the lateral separation of integral membrane proteins as it is happening in the case of some intermediate steps of the vesicular membrane trafficking. In the second type of membranes the bilayer part is ruffled and folded, and there the membrane skeletons play a role in the determination of the cortical tension. Here we explore in more detail the mechanical behavior of a cell with such properties of its boundary. The shape transformations are described which occur under the influence (i) of different external forces, i.e., when an originally spherical cell is aspirated into the micropipette or when such a cell is adsorbed on a flat surface, and (ii) of different internal forces on the cell boundary exerted by the cytoskeletal elements.     

Thylakoid membrane remodeling during state transitions in Arabidopsis

Adaptability of oxygenic photosynthetic organisms to fluctuations in light spectral composition and intensity is conferred by state transitions, short-term regulatory processes that enable the photosynthetic apparatus to rapidly adjust to variations in light quality. In green algae and higher plants, these processes are accompanied by reversible structural rearrangements in the thylakoid membranes. We studied these structural changes in the thylakoid membranes of Arabidopsis thaliana chloroplasts using atomic force microscopy, scanning and transmission electron microscopy, and confocal imaging. Based on our results and on the recently determined three-dimensional structure of higher-plant thylakoids trapped in one of the two major light-adapted states, we propose a model for the transitions in membrane architecture. The model suggests that reorganization of the membranes involves fission and fusion events that occur at the interface between the appressed (granal) and nonappressed (stroma lamellar) domains of the thylakoid membranes. Vertical and lateral displacements of the grana layers presumably follow these localized events, eventually leading to macroscopic rearrangements of the entire membrane network.     

Functional role of cardiolipin in mitochondrial bioenergetics

Cardiolipin is a unique phospholipid which is almost exclusively located in the inner mitochondrial membrane where it is biosynthesized. Considerable progress has recently been made in understanding the role of cardiolipin in mitochondrial function and bioenergetics. This phospholipid is associated with membranes designed to generate an electrochemical gradient that is used to produce ATP, such as bacterial plasma membranes and inner mitochondrial membrane. This ubiquitous and intimate association between cardiolipin and energy transducing membranes indicates an important role for cardiolipin in mitochondrial bioenergetic processes. Cardiolipin has been shown to interact with a number of proteins, including the respiratory chain complexes and substrate carrier proteins. Over the past decade, the significance of cardiolipin in the organization of components of the electron transport chain into higher order assemblies, termed respiratory supercomplexes, has been established. Moreover, cardiolipin is involved in different stages of the mitochondrial apoptotic process, as well as in mitochondrial membrane stability and dynamics. This review discusses the current understanding of the functional role that cardiolipin plays in several reactions and processes involved in mitochondrial bioenergetics. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.     

光合作用各部分反应间的动态衔接与协调

光合作用是地球上最重要的化学反应,由种类繁多、性质各异的反应步骤联系起来。要使光合机构在不断变化的环境中仍能适应运转,它需要随时协调各个部分反应间的关系。本文主要综述了光合能量转换过程中类囊体膜的动态结构变化、同化力组分调节、光合作用原初反应对光照变化的响应及能量转换反应与二氧化碳同化过程的配合等。     

Cytochrome b 6 f complex: Dynamic molecular organization,function and acclimation

The cytochrome b ₆ f complex occupies a central position in photosynthetic electron transport and proton translocation by linking PS II to PS I in linear electron flow from water to NADP⁺, and around PS I for cyclic electron flow. Cytochrome b ₆ f complexes are uniquely located in three membrane domains: the appressed granal membranes, the non-appressed stroma thylakoids and end grana membranes, and also the non-appressed grana margins, in contrast to the marked lateral heterogeneity of the localization of all other thylakoid multiprotein complexes. In addition to its vital role in vectorial electron transfer and proton translocation across the membrane, cytochrome b ₆ f complex is also involved in the regulation of balanced light excitation energy distribution between the photosystems, since its redox state governs the activation of LHC II kinase (the kinase that phosphorylates the mobile peripheral fraction of the chlorophyll a/b-proteins of LHC II of PS II). Hence, cytochrome b ₆ f complex is the molecular link in the interactive co-regulation of light-harvesting and electron transfer.The importance of a highly dynamic, yet flexible organization of the thylakoid membranes of plants and green algae has been highlighted by the exciting discovery that a lateral reorganization of some cytochrome b ₆ f complexes occurs in the state transition mechanism both in vivo and in vitro (Vallon et al. 1991). The lateral redistribution of phosphorylated LHC II from stacked granal membrane regions is accompanied by a concomitant movement of some cytochrome b ₆ f complexes from the granal membranes out to the PS I-containing stroma thylakoids. Thus, the dynamic movement of cytochrome b ₆ f complex as a multiprotein complex is a molecular mechanism for short-term adaptation to changing light conditions. With the concept of different membrane domains for linear and cyclic electron flow gaining credence, it is thought that linear electron flow occurs in the granal compartments and cyclic electron flow is localised in the stroma thylakoids at non-limiting irradiances. It is postulated that dynamic lateral reversible redistribution of some cytochrome b ₆ f complexes are part of the molecular mechanism involved in the regulation of linear electron transfer (ATP and NADPH) and cyclic electron flow (ATP only). Finally, the molecular significance of the marked regulation of cytochrome b ₆ f complexes for long-term regulation and optimization of photosynthetic function under varying environmental conditions, particularly light acclimation, is discussed.Abbreviations Chl chlorophyll - cyt cytochrome - PS Photosystem